Human Immunodeficiency Virus (HIV) affects millions of people worldwide, and the prevention of HIV remains a very high priority, even in an era of widespread antiretroviral treatment. In the United States, the Center for Disease Control (CDC) estimates that of all HIV-positive US residents, approximately one fifth are unaware of their status, and this small proportion is responsible for transmitting half the new infections each year [2]. Worldwide, the gap in prompt diagnosis and treatment is far greater. At the end of 2010, an estimated 34 million people were living with HIV worldwide, up 17% from 2001. Although the majority of new HIV infections continue to occur in sub-Saharan Africa, the CDC estimated that the annual incidence of HIV infection from 2008-2011 in the United States has remained stable at around 15-16/100,000, with over 40,000 new infections each year. Thus, it is an urgent global health priority to find a safe and potent HIV vaccine that would prevent HIV infection or blunt its initial impact prior to diagnosis, including both destruction of the gut CD4 pool [3] and high risk of transmission [4].
Live attenuated vaccines have proven to be highly efficacious in humans and in non-human primates (NHP) against certain viral diseases, such as a live attenuated simian immunodeficiency virus (SIV) based vaccine for preventing SIV infection. Unfortunately, due to safety risks associated with live attenuated HIV, such a strategy is not applicable for HIV human vaccine.
As an alternative to live attenuated viral vaccines, the use of replication incompetent recombinant viral vectors has been explored for vaccines and other types of gene therapy. In particular, replication incompetent recombinant adenoviral vectors, particularly adenovirus serotypes 2 and 5 (Ad2 and Ad5) have been extensively studied for gene delivery applications, including vaccination. Although such replication incompetent Ad5 vector-based vaccines have been shown to elicit protective immune responses in a variety of animal models, the utility of recombinant Ad5 vector-based vaccines for human immunodeficiency virus (HIV) and other pathogens is likely to be limited by the high seroprevalence of Ad5-specific neutralizing antibodies (NAbs) in human populations [17]. For example, in a seroepidemiology study of 4,381 subjects worldwide, it was observed that Ad5 NAb titers were nearly universal and high titer in sub-Saharan Africa, with the majority of individuals exhibiting Ad5 NAb titers >200 [14].
Even though Ad5 has high seroprevalence in humans, several HIV-1 vaccine efficacy trials have been conducted using vaccines based on recombinant Ad5 vector-based vaccines. These studies include the HVTN 502/STEP (Merck Ad5), HVTN 503/Phambili (Merck Ad5), and HVTN 505 (NIH VRC DNA/Ad5) HIV-1 vaccine efficacy trials. However, all three of these HIV-1 vaccine efficacy studies, which utilized nonreplicating Ad5 and DNA/Ad5 vaccines, showed no efficacy against HIV-1 infection. Moreover, a trend towards increased HIV-1 infection was observed in vaccinees with the Merck Ad5 vaccine from the STEP study as compared with placebos. Experience to date with replication incompetent vectors such as adenovirus subtype 5 for HIV vaccine has been disappointing, with failure to show benefit in several efficacy trials [5-8].
Accordingly, concerns regarding the safety of Ad5 vectors, particularly from the STEP study [8, 10], have led to the exploration of biologically substantially different Ad vectors from alternative serotypes as viral vaccine vectors [11-13]. One example of an alternative adenovirus serotype to Ad5 is Adenovirus serotype 26 (Ad26). Ad26 is a non-enveloped DNA virus that is a relatively uncommon virus in humans. Ad26 is not known to replicate in any other species. A number of surveys for adenovirus in different populations have shown it to be isolated only rarely, and even when isolated, seldom associated with symptoms. Experimental inoculation, likewise, showed little evidence for serious infection. See, e.g., [14, 27-43]. Thus, there is no evidence from observational studies that Ad26 causes clinical symptoms in healthy adults, and experimental data from an Ad26 challenge study also suggested that enteric Ad26 infection does not produce symptoms [44].
In terms of at least receptor usage, in vivo tropism, interactions with dendritic cells, innate immune profiles, adaptive immune phenotypes, and protective efficacy against SIV in rhesus monkeys, Ad26 has proven to be biologically very different from Ad5 [11, 12, 15, 19-22]. Moreover, the safety and immunogenicity of nonreplicating Ad26 vector in humans has been demonstrated (ClinicalTrials.Gov NCT01215149). Furthermore, many of the advantageous biological differences between Ad5 and Ad26, such as lower seroprevalance and low neutralizing antibody titers in humans are also present between Ad5 and Ad35.
Replication-incompetent Ad26 has been tested in a GLP toxicology study and three Phase I clinical trials with no significant pattern of adverse effects. Although replication incompetent viral vectors are preferred for gene therapy and related applications, such as vaccination, since replicating viral vectors can produce multiple copies of the virus, which can go on to infect other cells, setting of an infections cycle, there are some possible drawbacks to the use of replication incompetent viral vectors. One possible drawback of replication-incompetent viral vectors is that expression of the target gene to be delivered to the host from the viral vector can decrease following administration of the vector. Being unable to replicate or propagate in the host, the viral vector cannot produce any new copies that can subsequently be used to augment gene expression, requiring re-administration of the viral vector. If the same adenovirus serotype is re-administered to the host, the host may generate neutralizing antibodies to that particular adenovirus serotype, resulting in a serotype specific anti-adenovirus response. Such a serotype specific anti-adenovirus response may prevent effective re-administration of the viral vector, rendering it less effective as a vaccine or gene delivery vehicle.
Accordingly, there is a need in the art for new recombinant viral vectors that can be used as vaccine vectors that overcome certain disadvantages associated with replication-incompetent recombinant viral vectors. In particular, there exists a need for new recombinant viral vectors that can be used as vaccine vectors against infectious diseases, such as HIV infection. Such a vaccine preferably would be simple to administer, long-acting, with minimal adverse effects. In the case of an HIV vaccine, the HIV vaccine further would preferably be effective against a wide scope of the diversity of circulating types of HIV transmission, including the most frequent.